Magnetic components are used in switching power supplies for the storage of electrical energy in a magnetic field. Magnetic components comprise an electrical part (windings) and a magnetic core. Where there is just one winding, the magnetic component is an inductor and where there is more than one winding the magnetic component is generally a transformer.
A magnetic core is a piece of magnetic material with a high permeability used to confine and guide magnetic fields in electromagnetic devices such as transformers and inductors. Magnetic cores are typically made from a ferromagnetic such as ferrites. The high permeability, relative to the surrounding air, causes the magnetic field lines to be concentrated in the core material. The magnetic field is created by a coil of wire around the core that carries a current. The presence of the core can increase the magnetic field of a coil by a factor of several thousand over what it would be without the core.
The use of a magnetic core can enormously concentrate the strength and increase the effect of magnetic fields produced by electric currents. The properties of a device will depend on a number of factors including for example the geometry of the magnetic core, the amount of air gap in the magnetic circuit and the properties of the core material.
Depending on the application, a variety of different magnetic core shapes are available. One or more electrical windings are wound around one or more sections of the core. A bobbin may be used to form and retain the windings. Bobbins are typically formed from an insulating material such as plastic.
There are a variety of different core shapes known, for example: open core shapes, including “I”, “C” “E” and “U” cores which are so called because of their corresponding cross sectional shape; and closed core shapes, which may be formed by combining such open core shapes together.
The present application is directed to applications where the power supply is supplying of the order of less than 300 Watts. In such applications, the power supply may be housed in an adapter of the type which would be familiar to most laptop users. In such applications, minimising the size\weight of the power supply is generally desirable. At the same time because of the mass production nature of these supplies, using readily available core components is desirable for both cost and manufacturability.
To this end, in the context of such core components, the most common approach is to select an E-shaped core section are generally selected to form a closed magnetic core using either a second “E” shaped section or an “I” shaped section with the electric circuit wound around the resulting centre leg. The E-section core tends to be the most common type of core employed due to its shielding properties and the ability to support the structure mechanically.
A number of variations on the general E shaped cores are known including pot cores and EFD, ER and EP cores. For example, a pot core may be viewed as having a generally “E” shaped cross section albeit that it has been rotated somewhat to further enclose the centre leg between the outer legs.
Whilst the magnetic core is one part of the magnetic component, an equally important part is the electrical part. The electrical part provides conductive elements which form turns around the magnetic material referred to generally as windings. These windings may be in the form of stampings, rigid or flexible printed circuit boards or wound wire (on bobbins, or self-supporting). Optimising the winding structure is important in the context of getting best performance from the magnetic component. It will be appreciated that the definition of best performance will vary depending on the application and will generally involve a trade-off between different characteristics. For example, depending on the application, it may be desirable to minimise leakage inductance. Equally in other applications it may be desirable to have increased leakage inductance. For example, with switching power supplies, of the type to which the present application is directed, it is generally desirable to have compact magnetic components and low losses. At the same time there can be conflicting demands. For example, for a transformer employed in a mains powered switching power supply it is essential that sufficient isolation be provided between primary and secondary windings. It will be appreciated that magnetic components employed in switching power supplies are not generally comparable with those used for general AC transformer applications, i.e. mains frequencies of 50/60 Hz. Switching power supplies generally operate at frequencies above audio frequencies, i.e. above 20 kHz and so would have entirely different design characteristics. Thus, whilst iron laminate cores may be common in mains (non-switched) transformers, similar iron laminate cores (i.e. laminate cores having a thickness suited for use in mains transformers) would be considered entirely unsuitable in a switched power supply.
Thus in switching power supplies, the cores selected for magnetic components are generally formed from a suitable ferrite material. Most ferrites used for cores in switching power supply deployments have a relative permeability (μr) in the order of 500 or more. Low effective permeability is desired in many magnetic components, and accordingly in the case of ferrite materials, it is usual to introduce a gap in the magnetic path through the ferrite material.
There is a number of ways of forming air gaps with three legged cores using “E” shaped cores. For example, in FIG. 1 (a) a magnetic component 10A is provided with three air gaps formed by assembling an “E” core 11 and an “I” core 12 with a gap there between, thereby leaving air gaps between the “I” core and each leg of the “E” core 11, or as shown in FIG. 1(b) by assembling two “E” cores 11 with a gap there between to provide the core 10B.
However, the conventional solution in providing an “air” gap, as shown in the core 10C of FIG. 1(c) is to shorten the centre leg of a first “E” core 11a and to assemble this in combination with a second “E” core 11b. In this manner, the air gap sits in the middle of the coil which in turn would typically is wound around the centre leg, i.e. there is no gap in the outer legs. This is done so as to minimise fringing and reduce electromagnetic interference. The term “air” gap is generally used to refer to any gapped core as such even though the gap may not be filled by air but by nylon or some other non-saturable material (non-saturable being relative to the magnetic material used in the core).
Less conventionally, an “E” core 11a with a shortened leg might be combined with an “I” core 12 to provide a core 10D as shown in FIG. 1(d). Magnetic cores are however fragile and grinding operations to shorten a leg can produce an unreliable gap length, whilst at the same time introducing a step in the manufacturing process. Although cores prefabricated with a shortened leg are known, it will be appreciated that this limits the component designer's freedom to meet particular design objectives.
A further problem with the conventional approach of FIG. 1(c) is that a large fringe-field area exists which can cause significant loss in the conductive materials. This effect may be ameliorated somewhat using Litz-wire in windings, or by techniques involving the interchange of strands in printed-wire conductors. However, Litz windings whilst desirable for losses introduce other problems, for example the limited availability of adequate insulation to ensure isolation between primary and secondary windings. Nonetheless, even where Litz wire is employed considerable increases in AC-resistance can still occur due to presence of the air-gap. Recommended industry practice can be to keep wiring away from the gap, with the distance typically being several times the gap length. This creates difficulties and requires careful arrangement of the windings, which it will be appreciated, can complicate the designing of the winding arrangements.
The present application seeks to address one or more of the shortcomings set forth above and to provide a magnetic component which is suitable for use in a compact switching power supply where the power converted is of the order of less than 300 Watts.